High-resolution modal mapping in a complex shallow-water environment

Abstract

During March 1997, a modal mapping experiment (MOMAX) was conducted to measure the spatial variability of low-frequency sound propagation in shallow water [G. V. Frisk et al., J. Acoust. Soc. Am. 103, 3028(A) (1998)]. The fields created by cw sources in the frequency range 50–300 Hz were measured using an array of freely drifting hydrophone buoys equipped with GPS navigation. The resulting data consist of complex pressure mapped onto a high-resolution geospatial grid. Based on the tenets of normal mode theory, the wave number content of the field was extracted using a high-resolution implementation of the Hankel tranform technique [K. M. Becker et al., J. Acoust. Soc. Am. 103, 3029(A) (1998)]. Neglecting horizontal refraction, the resulting spectra can be mapped onto a corresponding spatial grid. Several modal maps are presented for various source/receiver configurations and frequencies with emphasis placed on the variablility of the wave numbers in space. These modal maps can then be used as the basis for inferring the local geoacoustic properties of spatially varying shallow-water waveguides. [Work supported by ONR.]